Before you start repair, discharge all capacitors in the amp because some may have DC 100V+ stored in them.
Test all the capacitors and diodes (if any) to see if any of them has shorted.
If that doesn’t do it you may have to do a “live” test.
For example, first disconnect all the outputs of the power supply transformer and turn on the amp. Since transformers rarely go bad, I expect the fuse to stay intact. Then you can reconnect the rectifying circuits back to the transformer and disconnect the output of the rectifying circuit, and test again. And continue adding more of the circuit back until the fuse is blown. Since you have the schematic this shouldn’t be too hard. Just be sure to unplug the amp and discharge the capacitors before you touch anything else.